What is the recommended antibiotic for beta-hemolytic strep (BHS) group B infections, particularly in pregnant women and patients with a history of allergies?

Antibiotic Susceptibility for Group B Streptococcus

Penicillin remains the drug of choice for Group B Streptococcus infections, as all GBS isolates worldwide remain 100% susceptible to penicillin with no confirmed resistance ever documented. 1, 2

First-Line Treatment Options

• Penicillin G is the preferred agent due to its narrow spectrum of activity, universal GBS susceptibility, and proven efficacy 1, 3

  • Dosing: 5 million units IV initially, then 2.5-3.0 million units IV every 4 hours 4, 1

• Ampicillin is an acceptable alternative with equivalent efficacy but broader spectrum activity 1, 3

  • Dosing: 2g IV initially, then 1g IV every 4 hours 1

• All GBS isolates remain susceptible to:

  • Penicillin (100% susceptibility) 1, 2, 5
  • Ampicillin (100% susceptibility) 1
  • Vancomycin (100% susceptibility) 5
  • Cefotaxime (100% susceptibility) 5
  • Chloramphenicol (100% susceptibility) 5

Alternative Antibiotics for Penicillin Allergy

For Non-Severe Penicillin Allergy (No History of Anaphylaxis)

• Cefazolin is the preferred alternative for patients without high-risk allergy symptoms 4, 1
  • Dosing: 2g IV initially, then 1g IV every 8 hours 1, 3
  • Cross-reactivity between penicillins and cephalosporins occurs in approximately 10% of patients with penicillin allergy 4, 2

For Severe Penicillin Allergy (History of Anaphylaxis, Angioedema, Respiratory Distress, or Urticaria)

The choice of antibiotic depends on susceptibility testing results:

• Clindamycin if the isolate is susceptible to both clindamycin and erythromycin 4

  • Dosing: 900 mg IV every 8 hours 1, 3
  • Critical caveat: If the isolate is erythromycin-resistant but clindamycin-susceptible, D-zone testing must be performed to detect inducible clindamycin resistance 4
  • Clindamycin can only be used if D-zone testing is negative 4

• Vancomycin if the isolate is resistant to clindamycin, demonstrates inducible clindamycin resistance, or if susceptibility is unknown 4, 1

  • Dosing: 1g IV every 12 hours 1, 3

Resistance Patterns and Clinical Implications

Resistance rates vary significantly by antibiotic class:

• Erythromycin resistance: 20.2% overall, with geographic variation (California 32%, Florida 8.5%) 5

  • Some studies report resistance rates as high as 23.1% 6

• Clindamycin resistance: 6.9% overall, with geographic variation (California 12%, Florida 2.1%) 5

  • Inducible clindamycin resistance occurs in 10.7% of isolates 6
  • Constitutive clindamycin resistance occurs in 14.9% of isolates 6

• Tetracycline resistance: High rates (74.5% for tetracycline, 72.4% for doxycycline in Group B streptococci) 7

Critical Testing Requirements

For penicillin-allergic patients at high risk for anaphylaxis, antimicrobial susceptibility testing is mandatory: 4

• Laboratories must perform clindamycin and erythromycin susceptibility testing 4
• D-zone testing is required for isolates that are erythromycin-resistant but clindamycin-susceptible 4, 2
• Clinicians must inform laboratories of the need for susceptibility testing to ensure proper testing is performed 4

Common Pitfalls to Avoid

• Never assume clindamycin susceptibility without testing in penicillin-allergic patients, as resistance rates range from 3-15% and are increasing 1, 2
• Do not use erythromycin alone for GBS infections due to increasing resistance rates 2
• Avoid cefazolin in patients with severe penicillin allergy (history of anaphylaxis, angioedema, respiratory distress, or urticaria) due to cross-reactivity risk 4
• For β-hemolytic streptococcal infections, treatment should continue for at least 10 days 8